The present invention relates to a method of checking the integrity of a communications network and, more particularly, to a method of detecting a loopback condition in the communications network.
A communication network may be populated by a number of switches, as shown in FIG. 1. The switches are typically interconnected by optical or electrical cables (xe2x80x9cfacilitiesxe2x80x9d) that carry communication traffic. In a time division multiplexed (TDM) scheme, facilities carry communication data for one call in a time slot (xe2x80x9ctrunkxe2x80x9d) multiplexed with data for other calls in other time slots. For two way communication, a call is assigned two trunks. A forward trunk on a first facility 10 carries data in a forward direction, such as from an originating switch 100 to a terminating switch 200; a return trunk on a second facility 20 carries data in a reverse direction, such as from the terminating switch 200 to the originating switch 100. By convention, the same trunks of the two facilities are assigned to the same call. For example, if trunk no. 2 of facility 10 is assigned as the forward trunk, trunk no. 2 of an associated facility 20 is assigned for the return trunk.
Control signals typically are used for call setup and tear down in communication networks (i.e., to establish call paths through the network during call setup and termination). Although communication traffic from an originating switch 100 is transmitted to a terminating switch 200 over trunks, the trunks do not carry control signals. To setup and terminate calls, an originating switch sends call setup information over an A LINK 30 to a signal transfer point (xe2x80x9cSTPxe2x80x9d) 300. The STP 300 communicates the call setup information to the terminating switch over a second A LINK 40. Through this out-of-band signaling, the originating switch 100 and terminating switch 200 confirm that each is operating and establish trunk assignments for the new call.
Out-of-band signaling does not permit the originating switch 100 or the terminating switch 200 to confirm the integrity of the facilities that interconnect them. Although the originating switch 100 and terminating switch 200 communicate over the A LINK, they would not be able to determine whether one or more of the facilities that interconnect them are damaged. If facility 50 is severed, for example, communications data placed on the facility 50 by the originating switch 100 would be lost; it would never reach the terminating switch 200. Although the terminating switch 200 would expect to receive communication traffic over the facility 50, the absence of communication traffic on the facility would not normally generate an alarm condition and/or a report.
To detect facility failures, it is known to conduct a continuity test in which, before carrying communication traffic, the originating switch 100 instructs the terminating switch 200 via the A LINK to switch all the data that it receives on the forward trunk to the return trunk. The originating switch 100 generates a test tone on the forward trunk and monitors the return trunk to detect the tone. The originating switch 100 determines that the facilities are operable when it detects the test tone on the return trunk. When it confirms that the facilities are operating properly, the originating switch 100 completes the call setup process.
The continuity test, while it is useful to detect certain facility failures such as open conditions, is vulnerable to other network failures, such as loopback conditions. A loopback condition occurs when a facility originates and terminates at the same switch, shown as 60 and 70 in FIG. 1. Most often, loopback conditions are caused by human error during facility installation or maintenance. The facilities are configured incorrectly. When a continuity test is run on a facility 60 that loops from a switch back to the same switch, the continuity test generates a xe2x80x9cfalse positive;xe2x80x9d the originating switch 100 detects the test tone on the xe2x80x9creturn trunkxe2x80x9d even though the forward trunk failed to reach the terminating switch.
Although a loopback facility 60 cannot carry data between the originating switch 100 and the terminating switch 200, the continuity test cannot identify this type of equipment failure. The limitations of the continuity test result in unsuccessful calls and customer dissatisfaction.
Accordingly, there is a need in the art for a test protocol in communications networks that detect the presence of loopback conditions. Further, there is a need in the art for a continuity test protocol that tests for loopback conditions and maintains the viability of the continuity test protocol.
The disadvantages of the prior art are alleviated to a great extent by a test protocol that tests for loopback conditions according to the following method: An originating switch engages a normal call setup operation to communicate with and confirm the operation of the terminating switch. The originating switch generates a test tone on the forward trunk. Unlike the continuity test, the originating switch does not instruct the terminating switch to switch the forward trunk to the return trunk. If the originating switch monitors the return trunk and detects the test tone, the originating switch fails the facility due to the presence of a loopback condition.
The loopback test protocol complements and works with the continuity test. The tests may be run in succession to detect facility integrity and the absence of loopback errors. For example, while setting up on a first call, the continuity test may confirm the viability of the facility through the known continuity test. Then, on a subsequent call, the loopback test procedure may be run. By alternating the continuity and loopback tests, the present invention detects both error conditions.